Fixed resistor unit and process of forming the same



Oct. 23, 1934. L. E. POWER 1,978,323

FIXED RESISTOR UNIT AND PROCESS OF FORMING THE SAME Filed July 18 1931 INVENTOR Lax/P5175 5 E FEW BY Jaw ATTORNEY Patented Oct. 1934 PATENT OFFICE FIXED RESISTOR UNIT AND PROCESS OF FORMING THE SAME Laurence E. Power, Milwaukee, Wis., assignor, by I mesne assignments, to Allen-Bradleycompany,

Milwaukee, Win, a corporation of Wisconsin Application July 18, 1931, Serial No. 551,608

12 Claims.

This invention relates to fixed resistance units and refers more particularly to the contact portions thereof.

It is of paramount importance, in electrical I circuits including a resistance unit,or unitsand where conditions must remain absolutely constant, that the junction between the electrical resistance unit and the conductor connecting it in the circuit, have a very low resistance value and that this low resistance value should not change. A particular example of such a condition is the radio receiving circuit in which there are numerous resistance units. In each instance the junction between the resistance unit and its conductors must be of low resistance and must be constant to insure successful operation of the circuit. The requirement for low resistance contacts for fixed resistors has been recognized in the past and numerous attempts have been made to meet :0 this need. The Patent No. 1,816,194 issued to L. E. Power July 28, 1931, discloses one method of providing a resistor unit with low resistance ends or contact portions, and while the unit and process of forming the same disclosed in this patent as has proven successful, there has since arisen an increasing demand for more economical manufacture. r

It is therefore an object of this invention to, provide a resistance unit of novel construction and a process of forming the same which permits its manufacture at a relatively low cost.

Another object of this invention is to provide a fixed resistor unit and a method of forming the same in which contact portions of low resistance are formed integral with thebody of the unit.

Another object of this invention is to provide a resistor unit of the character described which is equipped with low resistance ends or contact portions which remain constant in their low value of resistance through long periods of service.

A further object of this invention resides in the provision of a resistor unit, the body of which is formed of ceramic and conductive materials and has particles of conducting material embedded in the end portions thereof;

With the above and other objects in view which will appear as the description proceeds, this invention resides in the novel construction, combination and arrangement of parts substantially as so hereinafter described and more particularly defined by the appended claims, it being understood that such changes in the precise embodiment of the hereindisclosed invention may be" made as come within the scope of the claims. a In the accompanying drawing, two complete examples of the physical embodiment oi this invention are illustrated constructed according to the best modes so far devised for the practical application of the principles thereof, and in which: 0

Figure 1 is a side elevation view of a completed resistor unit constructed in accordance with this invention and provided with metal end caps and conductors soldered to the end caps;

Figure 2 is a longitudinal section view through the resistor unit, per se;

Figure 3 is a view similar to Figure 2 of a slightly modified form of resistor unit;

Figure 4 is an enlarged fragmentary view of one end portion of the unit shown in Figure 3 before it is completed; and

Figure 5 is a view similar to Figure 4 showing the unit end after the next operation.

Referring now more particularly to the accompanying drawing, the numeral 5 represents the body portion of a fixed resistor unit constructed in accordance with this invention. As shown, the body portion is preferably rod shaped and is formed from a ceramic material, which in a broad sense, is a clay-like material becoming porcelain on being fired. This clay-like material may consist of a mixture of the following ingredients, substantially in the proportions given:

40% ceramic grade Durox" 30% plastic "Vitroxf 30% Lake County kaolin Throughout the entire body portion 5 are part1 cles of carbon black 6, the percentage of which determines the resistance value of the unit.

The extreme end portions 7 of the body contain a comparatively high percentage of carbon black and therefore provide contact portions of good electrical conductivity. To these end portions of good electrical conductivity metal end caps 8 are secured, and if desired conductor leads 9 may be soldered or otherwise secured to the end caps.

In Figure 3, a slightly modified construction is illustrated and .in this instance the body portion 5' is provided with an axial bore 10 extending through-out its entire length. The body portion 5' however contains no carbon black, and the conductivity of the unit in this instance is provided by acore 11 filling the bore 10 and formed of the same ceramic material of which the body portion is formed, but containing carbon black, the percentage of which likewise determines the resistance value of the unit.

As in the embodiment illustrated in Figure 2, the end portions also contain a relatively high percentage of carbon black to provide good electrical contact portions.

In the manufacture of the units described, the ceramic material, after having been prepared either with or without the carbon black, in a moist 'state, is"extruded, pressed, or otherwise shaped into a long rod. The rod is dried at approximately room temperature (68 F.) for three or four hours and then cut into appropriate lengths. The pieces thus provided are preferably of the desired shape of the finished unit and are baked at a tem-- perature to produce porosity. A temperature of between 1400 degrees F. and 1800 degrees F., has been found to produce maximum porosity where the unit is formed of a ceramic material such as described.

The porous nature of the units at this stage is utilized to permit an impregnation or saturation of its end portions or any other parts thereof which may be used as a contact, with additional carbon black or other conducting material. The impregnation of these portions with carbon black is brought about preferably by immersing them in a. carbonaceous varnish-like liquid which contains a relatively high percentage of fixedcarbon. A phenol condensation product varnish dissolved in denatured alcohol has been found suitable for this purpose, but any fugitive carrier of current conducting material may be used.

If the solvent or fugitive carrier for the conducting material is water, it is esential that the heat treatment to produce porosity be at-a temperature of between 1400 degrees F. and 1800 degrees F., as water disintegrates the ceramic material unless it has been heated to at least 1400 degrees F. When other solvents are used, a lower temperature may be employed to produce porosity. Good results may be had with temperatures as low as 250 degrees F., although it is understood that greater porosity and consequently a better unit is obtained with higher temperatures.

After the pieces have had their contact or end portions immersed, they are subjected to a temperature of approximately 2200 F. to 2400 F. at which temperature the carbon or other current conducting material is freed from its carrying agent and becomes fixed in the pores of the contact portions of the unit. This baking process also vitrifies the unit as a whole to a glass-like hardness.

Inasmuch as the unit is porous throughout its entire extent at the time of its immersion, the varnish-like material, with the free carbon or other conducting material contained therein, saturates the internal structure of the unit end portions to a. substantial depth. The resistance value of the finished end or contact portions is comparable to that of substantially pure carbon, and inasmuch as the end contact portions are integral with the body of the unit a mechanically good structure is obtained.

The fact that the carbon or other conducting material saturates the internal structure of the unit contact portions to a substantial depth permits grinding to fit the metal end caps if necessary, without increasing the resistance of the end portions.

In lieu of the carbonaceous varnish-like liquid, the end contact portions of the unit may be immersed in solutions of copper bromide, silver nitrate, nickle compounds, or combinations of one of such solutions with carbon forming compounds. In this instance, the remainder of the process is identical with that described, and the vitrifying temperature similarly fixes the metal or metal saurated cone shaped center.

and carbon in the pores of the unit" contact portions. I I i If it .is found jthat the impregnation of the resistor unit with. carbon, metal or other currentconducting material causes a softening or weakening of the end structure, which isundesirable,

a fluxsuch asboric acid may be added to the liquid with which the end portions are saturated, to overcome this condition. The metal caps may be dispensed with if desired and the end portions of the unit coated with metal or any other suitable mate rlal, or even .the coating of metal may be dispensed with and the unit inserted directly into connection clips.

It is noted that the contact end portions are not saturated throughout but that there is an un- This fact enables high resistance values to be obtained with comparatively short units, with the structure shown in Figure 3. In this instance, the core of the unit which forms the conducting path, extends to the extreme endsof the unit andthe hollow centers of the contact portions preclude contact between the core and the contact portions except at the extreme ends of the core, so that practically the entire length of the core is available for effective resistance.

During the cutting of the rod intoshort pieces, the structure of the end portions, especially in that modification shown in Figure 3, is apt to crack as depicted in Figure 4. These cracks, if left open permit the low resistance contact material to penetrate into the unit and make contact with the core at varying distances from the unit ends. This condition is obviously objectionable as it reduces the length of the effective resistance path and destroys uniformity.

To overcome this objection the ends of the resistance core may be upset in any desired manner, as at 12 in Figure 5. In so doing, the resistance core end seals the cracks and prevents the low resistance contact material from entering the same and permeating the unit end portions beyond the desired degree. The upsetting of the resistance core ends is done directly after the rod is out into small pieces.

From the foregoing description taken in connection with the acompanying drawing, it will be readily apparent to those skilled in the art to which an invention of this character appertains, that resistor units constructed in accordance with this invention may be produced at a relatively low cost and that their glass like hardness makes them exceptionally durable.

The important feature of this invention however, resides in the novel formation of the contact portions and while in the foregoing description these contact portions are defined as part of a resistor unit, it is readily apparent that the spirit of the invention includes the formation of any electrical contact body in the manner set forth irrespective of its application. I v

What I claim as my invention is: s a. a, i

1. The process of forming a fixed electrical resistor unit which comprises, forming a vitrifiable material into a body of the desired size and shape of the finished unit, heat treating said body at a temperature to produce porosity, immersing the end portions of the body in a fugitive carrier of current conducting elements, and subjectingthe body to a temperature higher than that employed to produce porosity and at which the current conducting elements are freed from said fugitive carrier and become fixed in the pores of the body end portions.

2. The process of forming a fixed electrical resistor unit which comprises, forming a quantity of substantially plastic ceramic material into "a body of the desired shape of the completed unit, subjecting said body to heat at a temperature to increase its porosity, applying a current conductingmaterialtoaportionofthebodyinsucha manner that the current conducting material penetrates into the pores of said portion of the body, and subjecting the body with the current conducting material applied thereto to a vitrlfying heat so that the entire body with the current conducting material in said portion thereof assumes a glass like hardness.

3. The process of forming a fixed electrical resistor unit which comprises, forming a tubular body of nonconducting ceramic material, filling the interior of said body to the extreme ends of the tubular body with material having a predetermined degree of electrical conductivity, and in impregnating the end portions of the tubular body with low resistance current conducting material to contact with the conducting material filling the interior of the tubular body and provide contact portions therefor.

4. The process of forming a fixed electrical resistor unit which comprises forming a tubular body of nonconducting ceramic material, filling the interior of said body with a material having a predetermined degree of electrical conductivity to the extreme ends of the tubular body, immersing the end portions of the tubular body in a fugitive carrier of electrical conducting material, and in subjecting the body to a temperature at which the current conducting material is freed from said fugitive carrier and becomes fixed in the pores of the tubular body end portions to provide contact portions of low electrical resistance inelectrical contact with the conducting material filling the interior of the body.

5. A fixed electrical resistor unit comprising, a tubular rod like body of insulating material, a coreof non-conducting and conducting materials in predetermined proportions extending throughout the entire length of the tubular body and to the extreme ends thereof, and contact portions at the ends of the body containing a large percentage of conducting material which fills the pores of the body end portions and thus contacts with the ends of the material filling the interior of the body.

8. The process of forming a fixed electrical resistor unit which comprises forming a tubular body of non-conducting material, providing a core of material having a predetermined degree of conductivity for said tubular body which extends to the extreme ends of the tubular body, upsetting the ends of the core to seal any fissures in the tubular body ends, and impregnating the end portions of the tubular body with low resistance current conducting material to contact with tthe e(sore ends and provide contact portions er or.

'LTheprocessoimakinganelectricalcontact,

which comprises providing a body of ceramic material, heat treating the body to produce porosity, saturating the body with a fugitive carrier of conductingmaterial to a substantial depth, and again heat treating the body at a higher temperature than the first heat treatment to free the current conducting material from said fugitive carrier and fix the same in the pores of the body.

8. The process of making a good electrical conductor of a body of non-conducting ceramic material which comprises, heat treating said body to produce porosity, immersing the body in a fugitive carrier of conducting material so that said fugitive carrier and the conducting material contained therein saturates the body to a'substantial depth, and again heat treating the body at a higher temperature than the first heat treatment to drive off the fugitive carrier and fix the current conducting material in the pores of the body.

9. The hereindescribed process of making a fixed electrical resistor unit, which comprises forming a plastic material into a body of the desired size and shape of the finished unit, heat porosity, immersing the end portions of the body in a fugitive carrier of current conducting material, and subjecting the body to a temperature higher than the first temperature to harden the unit and fix the current conducting material in its 10. In the process of making a fixed electrical resistor unit, the steps of heat treating the unit to increase its porosity, impregnating certain portions of the unit with a fiuid substance containing a conducting materialgand subjecting the unit to a second heat treatment at a higher temperature to freev the current conducting material from said substance and harden the unit.

11. The hereindescribed process of making a 118 fixed electrical resistor unit, which comprises forming a plastic ceramic material into a body of the desired size and shape of the finished unit, heat treating said body at a temperature to produce porosity, immersing the end portions of the 120 body in a fugitive carrier of current conducting material, and subjecting the body to a temperature higher than the first temperature to vitrify the unit and fix the current conducting material in its pores.

12. The process of making a fixed electrical resistor unit, which comprises forming a plastic material into a body of the desired size and shape, subjecting the body to a temperature ranging from between 250 degrees F. to 1800 degrees I". m 1 

